On the propensity of magnetism in 3d transition-metal-MgCNi3 alloys

The effects of disorder and incipient magnetism in MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ (T≡Fe$\mathrm{T}\equiv \mathrm{Fe}$, Co or Cu) alloys are studied using coherent-potential approximation and Ginzburg–Landau coefficients. The first-principles, local-density-functional-based calculations for substitutionally disordered Fe and Co impurities in the Ni sub-lattice of MgCNi₃, in low concentrations, show that incipient magnetism resides in these materials. The overestimation of the calculated magnetic properties points to the limitations of the local-density approximation. However, using a phenomenological approach based on Ginzburg–Landau coefficients and the fixed-spin moment method, we show that MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC(_{Ni0.95Fe0.05)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.95}{\mathrm{Fe}}_{0.05}{)}_3$ and MgC(_{Ni0.90Co0.10)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.90}{\mathrm{Co}}_{0.10}{)}_3$ alloys.     

The influence of 3d-impurities on magnetic and transport properties of CoSiB metallic glasses

Temperature dependencies of magnetic susceptibility and electric resistivity of Co-based metallic glasses (MGs) of the general composition _Co(72-x)Mex(Si,B₎₂₈${\mathrm{Co}}_{(72-x)}{\mathrm{Me}}_x(\mathrm{Si},\mathrm{B}{)}_{28}$(Me=Fe,Cr,Si:B=18:10)$(\mathrm{Me}=\mathrm{Fe},\mathrm{Cr},\mathrm{Si}:\mathrm{B}=18:10)$ have been studied up to 950 K. The studied MGs were found to be ferromagnets at the room temperature and their Curie point _TC$T_{\mathrm{C}}$ ranges within 260–560 K depending on the dopant contents. At the temperatures higher than _TC$T_{\mathrm{C}}$, a wide paramagnetic region exists. The regularities of magnetic moment variation upon Cr doping evidence a formation of antiferromagnetic clusters, which determine the anomalous behavior of resistivity.     

Parameters in a class of leptophilic dark matter models from PAMELA,ATIC and FERMI

In this work we study a class of leptophilic dark matter models, where the dark matter interacts with the standard model particles via the U_(1)Li−Lj$U{(1)}_{L_i-L_j}$ gauge boson, to explain the e^±$e^{\pm }$ excess in cosmic rays observed by ATIC and PAMELA experiments, and more recently by Fermi experiment. There are three types of U_(1)Li−Lj$U{(1)}_{L_i-L_j}$ models: (a) U_(1)Le−Lμ$U{(1)}_{L_e-L_{\mu }}$, (b) U_(1)Le−Lτ$U{(1)}_{L_e-L_{\tau }}$, and (c) U_(1)Le−Lτ$U{(1)}_{L_e-L_{\tau }}$. Although ATIC or Fermi data are consistent with PAMELA data separately, ATIC and Fermi data do not agree with each other. We therefore aim to identify which of the three models can explain which data set better. We find that models (a) and (b) can give correct dark matter relic density and explain the ATIC and PAMELA data simultaneously recur to the Breit–Wigner enhancement. Whereas model (c) with a larger Z^′$Z^{\prime }$ mass can explain Fermi and PAMELA data simultaneously. In all cases the model parameters are restricted to narrow regions. Future improved data will decide which set of data is correct and also help to decide the correct dark matter model.     

Effects of Gd substitution on the structural and magnetic properties of strontium hexaferrites

The effect of Gd substitution in M-type strontium hexaferrites has been examined in two series of samples, (_Sr1-xGdx)O·5.25$({\mathrm{Sr}}_{1-x}{\mathrm{Gd}}_x)\mathrm{O}·5.25$_Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$ and _{Sr1-xGdxFe12-xCoxO19}${\mathrm{Sr}}_{1-x}{\mathrm{Gd}}_x{\mathrm{Fe}}_{12-x}{\mathrm{Co}}_x{\mathrm{O}}_{19}$, both prepared by the ceramic method, where x=0–0.40$x=0–0.40$. The samples have been characterized by XRD, VSM and SEM-EDAX techniques. All substituted samples present primarily the hexaferrite structure. Sample (_{Sr0.95Gdx0.05})O·5.25$({\mathrm{Sr}}_{0.95}{\mathrm{Gd}}_{x0.05})\mathrm{O}·5.25$_Fe2O3${\mathrm{Fe}}_2{\mathrm{O}}_3$ is single phase. Formation of impurity phases is affected by stoichiometry and presence of Co. In Sr–Gd samples, coercivity showed a maximum value of 305 kA/m (3.8 kOe) for x=0.20$x=0.20$, while remanence and saturation magnetization did not decrease. Coercivity and magnetization in the Sr–Gd–Co series decreased steadily with substitution degree.     

Multiple-vehicle collision induced by a sudden stop in traffic flow

We study the dynamic process of the multiple-vehicle collision when a vehicle stops suddenly in a traffic flow. We apply the optimal-velocity model to the vehicular motion. If a vehicle does not decelerate successfully, it crashes into the vehicle ahead with a residual speed. The collision criterion is presented by _vi(t)/Δ_xi(t)→∞$v_i(t)/\mathrm{\Delta }{x}_i(t)\to \infty$ if Δ_xi(t)→0$\mathrm{\Delta }{x}_i(t)\to 0$ where _vi(t)$v_i(t)$ and Δ_xi(t)$\mathrm{\Delta }{x}_i(t)$ are the speed and headway of vehicle i at time t. The number of crumpled vehicles depends on the initial velocity, the sensitivity, and the initial headway. We derive the region map (or phase diagram) for the multiple-vehicle collision.     

Comment on reparametrization invariance of quark–lepton complementarity

We study the complementarity between quark and lepton mixing angles (QLC), the sum of an angle in quark mixing and the corresponding angle in lepton mixing is π/4$\pi /4$. Experimentally in the standard PDG parametrization, two such relations exist approximately. These QLC relations are accidental which only manifest themselves in the PDG parametrization. We propose reparametrization invariant expressions for the complementarity relations in terms of the magnitude of the elements in the quark and lepton mixing matrices. In the exact QLC limit, it is found that |_Vus/_Vud|+|_Ve2/_Ve1|+|_Vus/_Vud||_Ve2/_Ve1|=1$|V_{us}/V_{ud}|+|V_{e2}/V_{e1}|+|V_{us}/V_{ud}||V_{e2}/V_{e1}|=1$ and |_Vcb/_Vtb|+|_Vμ3/_Vτ3|+|_Vcb/_Vtb||_Vμ3/_Vτ3|=1$|V_{cb}/V_{tb}|+|V_{\mu 3}/V_{\tau 3}|+|V_{cb}/V_{tb}||V_{\mu 3}/V_{\tau 3}|=1$. Expressions with deviations from exact complementarity are obtained. Implications of these relations are also discussed.     

Partition function zeros of the antiferromagnetic Ising model on triangular lattice in the complex temperature plane for nonzero magnetic field

The grand partition functions Z(T,B)$Z(T,B)$ of the Ising model on L×L$L\times L$ triangular lattices with fully periodic boundary conditions, as a function of temperature T and magnetic field B  , are evaluated exactly for L<12$L<12$ (using microcanonical transfer matrix) and approximately for L?12$L?12$ (using Wang–Landau Monte Carlo algorithm). From Z(T,B)$Z(T,B)$, the distributions of the partition function zeros of the triangular-lattice Ising model in the complex temperature plane for real B≠0$B\ne 0$ are obtained and discussed for the first time. The critical points aN(x)$a_N(x)$ and the thermal scaling exponents yt(x)$y_t(x)$ of the triangular-lattice Ising antiferromagnet, for various values of x=e−2βB$x=e^{-2\beta B}$, are estimated using the partition function zeros.     

Discrete gauge symmetries in discrete MSSM-like orientifolds

Motivated by the necessity of discrete _ZN${\mathbb{Z}}_N$ symmetries in the MSSM to insure baryon stability, we study the origin of discrete gauge symmetries from open string sector U(1)$U(1)$?s in orientifolds based on rational conformal field theory. By means of an explicit construction, we find an integral basis for the couplings of axions and U(1)$U(1)$ factors for all simple current MIPFs and orientifolds of all 168 Gepner models, a total of 32 990 distinct cases. We discuss how the presence of discrete symmetries surviving as a subgroup of broken U(1)$U(1)$?s can be derived using this basis. We apply this procedure to models with MSSM chiral spectrum, concretely to all known U(3)×U(2)×U(1)×U(1)$U(3)\times U(2)\times U(1)\times U(1)$ and U(3)×Sp(2)×U(1)×U(1)$U(3)\times \mathit{Sp}(2)\times U(1)\times U(1)$ configurations with chiral bi-fundamentals, but no chiral tensors, as well as some SU(5)$\mathit{SU}(5)$ GUT models. We find examples of models with _Z2${\mathbb{Z}}_2$ (R-parity) and _Z3${\mathbb{Z}}_3$ symmetries that forbid certain B and/or L violating MSSM couplings. Their presence is however relatively rare, at the level of a few percent of all cases.     

Determination of the Landau–Lifshitz damping parameter by means of complex susceptibility measurements

A new experimental method for the determination of the Landau–Lifshitz damping parameter, α$\alpha$, based on measurements of the frequency and field dependence of the complex magnetic susceptibility, χ(ω,H)=χ^′(ω,H)-iχ^″(ω,H)$\chi (\omega ,H)={\chi }^{\prime }(\omega ,H)-\mathrm{i}{\chi }^{″}(\omega ,H)$, is proposed. The method centres on evaluating the ratio of f_max/f_res, where f_res is the resonance frequency and f_max is the maximum absorption frequency at resonance, of the sample susceptibility spectra, measured in strong polarizing fields. We have investigated three magnetic fluid samples, namely sample 1, sample 2 and sample 3. Sample 1 consisted of particles of Mn_0.6Fe_0.4Fe₂O₄ dispersed in kerosene, sample 2 consisted of magnetite particles dispersed in Isopar M and sample 3 was composed of particles of Mn_0.66Zn_0.34Fe₂O₄ dispersed in Isopar M  . The results obtained for the mean damping parameter of particles within the magnetic fluid samples are as follows: 〈α(_{Mn0.6Fe0.4Fe2O4})〉=0.057$〈\alpha ({\mathrm{Mn}}_{0.6}{\mathrm{Fe}}_{0.4}{\mathrm{Fe}}_2{\mathrm{O}}_4)〉=0.057$ with the corresponding standard deviation SD=0.0104$\mathrm{SD}=0.0104$; 〈α(_Fe3O4)〉=0.1105$〈\alpha ({\mathrm{Fe}}_3{\mathrm{O}}_4)〉=0.1105$ with the corresponding standard deviation, SD=0.034$\mathrm{SD}=0.034$ and 〈α(_{Mn0.66Zn0.34Fe2O4})〉=0.096$〈\alpha ({\mathrm{Mn}}_{0.66}{\mathrm{Zn}}_{0.34}{\mathrm{Fe}}_2{\mathrm{O}}_4)〉=0.096$ with the corresponding standard deviation, SD=0.037$\mathrm{SD}=0.037$.     

Topology and phase transitions II. Theorem on a necessary relation

In this second paper, we prove a necessity theorem about the topological origin of phase transitions. We consider physical systems described by smooth microscopic interaction potentials VN(q)$V_N(q)$, among N   degrees of freedom, and the associated family of configuration space submanifolds _{Mv}v∈R${\{M_v\}}_{v\in \mathbb{R}}$, with Mv={q∈RN|VN(q)?v}$M_v=\{q\in {\mathbb{R}}^N|V_N(q)?v\}$. On the basis of an analytic relationship between a suitably weighed sum of the Morse indexes of the manifolds _{Mv}v∈R${\{M_v\}}_{v\in \mathbb{R}}$ and thermodynamic entropy, the theorem states that any possible unbound growth with N   of one of the following derivatives of the configurational entropy S(−)(v)=(1/N)logMv_∫dNq$S^{(-)}(v)=(1/N)\log {\int }_{M_v}{d}^{N}q$, that is of |k^∂S(−)(v)/∂vk|$|{\partial }^k{S}^{(-)}(v)/\partial v^k|$, for k=3,4$k=3,4$, can be entailed only by the weighed sum of Morse indexes. Since the unbound growth with N of one of these derivatives corresponds to the occurrence of a first- or of a second-order phase transition, and since the variation of the Morse indexes of a manifold is in one-to-one correspondence with a change of its topology, the Main Theorem of the present paper states that a phase transition necessarily stems from a topological transition in configuration space. The proof of the theorem given in the present paper cannot be done without Main Theorem of paper I.     

Parity doubling among the baryons

We study the evidence for and possible origins of parity doubling among the baryons. First we explore the experimental evidence, finding a significant signal for parity doubling in the non-strange baryons, but little evidence among strange baryons. Next we discuss potential explanations for this phenomenon. Possibilities include suppression of the violation of the flavor singlet axial symmetry (U(1_)A$U(1{)}_A$) of QCD, which is broken by the triangle anomaly and by quark masses. A conventional Wigner–Weyl realization of the SU(2_)L×SU(2_)R$\mathit{SU}(2{)}_L\times \mathit{SU}(2{)}_R$ chiral symmetry would also result in parity doubling. However this requires the suppression of families of chirally invariant   operators by some other dynamical mechanism. In this scenario the parity doubled states should decouple from pions. We discuss other explanations including connections to chiral invariant short distance physics motivated by large _Nc$N_c$ arguments as suggested by Shifman and others, and intrinsic deformation of relatively rigid highly excited hadrons, leading to parity doubling on the leading Regge trajectory. Finally we review the spectroscopic consequences of chiral symmetry using a formalism introduced by Weinberg, and use it to describe two baryons of opposite parity.     

Vacuum phase transition at nonzero baryon density

It is argued that the dominant contribution to the interaction of quark–gluon plasma at moderate T?Tc$T?T_c$ is given by the nonperturbative vacuum field correlators. Basing on that nonperturbative equation of state of quark–gluon plasma is computed and in the lowest approximation expressed in terms of absolute values of Polyakov lines for quarks and gluons Lfund(T),Ladj(T)=(Lfund)^9/4$L_{\mathrm{fund}}(T),L_{\mathrm{adj}}(T)={(L_{\mathrm{fund}})}^{9/4}$ known from lattice and analytic calculations. Phase transition at any μ   is described as a transition due to vanishing of one of correlators, DE(x)$D^E(x)$, which implies the change of gluonic condensate ΔG₂$\mathrm{\Delta }{G}_2$. Resulting transition temperature Tc(μ)$T_c(\mu )$ is calculated in terms of ΔG₂$\mathrm{\Delta }{G}_2$ and Lfund(Tc)$L_{\mathrm{fund}}(T_c)$. The phase curve Tc(μ)$T_c(\mu )$ is in a good agreement with lattice data. In particular Tc(0)=0.27$T_c(0)=0.27$; 0.19; 0.17 GeV$0.17\text{ GeV}$ for nf=0,2,3$n_f=0,2,3$ and fixed ΔG₂=0.0035 GeV⁴$\mathrm{\Delta }{G}_2=0.0035{\text{ GeV}}^4$.